1. Field of the Invention
This invention relates generally to a surface acoustic wave filter and more particularly concerns a broad-band band pass filter with a selected frequency response, comprising a thin film device in which surface acoustic waves are coupled from a transmitting interdigital transducer through reflectors to a receiving interdigital transducer.
2. Description of the Prior Art
In communication and electronic equipment, particularly, VHF and UHF television sets, radios, and wireless communication equipment, high frequency filters are indispensable.
As a high frequency filter, a filter is used wherein a surface acoustic wave is piezo-electrically generated and then detected. That is, a surface acoustic wave filter has been developed as a substitute for a conventional filter which is a combination of lamped constant elements such as coils, capacitors, etc. Various types of surface acoustic wave filters have been suggested so far according to the application requirements, and some of them are already used in television sets, etc.
The surface acoustic wave filter in which there are provided on a piezo-electric substrate two interdigital transducers each of which consists of thin film finger electrodes, is based on the fact that if the filter satisfies relation f=v/2L, then the filter transmits signals most efficiently. Where, L indicates the spacing distance between the abovementioned film finger electrodes, "v" is a velocity in synchronism with the spacing distance, and "f" is a frequency of signals to be transmitted. The filter of this type can be made more compact especially for use of signals from tens to several micro-meters in wave length in VHF and UHF frequency band. Further, it can be formed as a planer circuit in which thin film conductors are placed on a tiny chip surface. Because of its possibility of an integrated circuit, that is, compactness and its stable operation, great expectation has been placed on the development of the surface acoustic wave filter.
Surface acoustic wave filters are grouped into two broad classes, transversal and resonant filters. The former has a disadvantage that the loss is relatively large, since 50% of the energy in a surface acoustic wave converted through a transducer is absorbed into the acoustic material. On the other hand, the latter has an advantage that the loss is relatively small, since the energy converted to a surface acoustic wave is enclosed on the substrate surface. However, the latter has also a disadvantage that it is usually difficult to obtain a desired frequency response due to the frequency characteristic of the reflector. More specifically, although it is desirable for the filter frequency response to be flat, a resonant type of surface acoustic wave filter tends to fluctuate or ripple in the frequency response.
The inventors have proposed a surface acoustic wave filter with a low loss and a wide-band frequency response, prior to the present invention, in the Y. Kinoshita et al. U.S. application Ser. No. 18331, entitled "Surface Acoustic Filter". The surface acoustic wave filter described in the above U.S. application is of the same type as that in FIG. 1 in which a transmitting interdigital transducer 1 and a receiving interdigital transducer 2 are provided on a piezo-electric substrate 9; a group of reflecting electrodes 6 are positioned at the respective one sides of the transducers; and a directional multi-stripe coupler 7 is provided between the transducers 1 and 2 and the reflecting electrode group 6 for coupling of acoustic energy of a transmission signal to the receiving transducer.
The surface acoustic wave filter of the above-mentioned construction has been already known as a narrow band filter, for example, in the U.S. Pat. No. 3,886,504 and in an article entitled "Some Study on SAW Resonantor and Multiple-Mode Filters" by Y. Suzuki, H. Shimizu et al, in the Ultrasonic Symp. Proc., 1976, p. 297. However, in accordance with the theory known hitherto, it is impossible to get an ideal flat and broad-band frequency response. In other words, ripples will appear in the frequency response. The present inventors considered that such ripples result from the fact that multiple reflections occurs between the opposed reflecting electrode groups through the transducers. Therefore, in the surface acoustic wave filter according to the present invention, the above-mentioned multi-stripe coupler comprises a 3 dB directional coupler. It has been found that in the surface acoustic wave filter suggested in U.S. application Ser. No. 18331 by the same inventors, ripples can be remarkably reduced, leaving small amount of ripples in the frequency response. In order to eliminate such remaining ripples completely, the filter will require the addition of matching coils as inductance elements to the input and output signal circuits. However, the addition of external coils to the surface acoustic wave filter means reduction of the effectiveness of the filter.